What are you worried about, exactly? Unless I'm missing something, the worst-case scenario here is that, at a pre- or very-early-5.4 level of gearing, Combat will be as shunned by progression raiders as it already is right now.

Once you grab even the LFR version of AoC (which drops from the final boss of the first "wing" of SoO -- so, pretty early in the raid) you'll catch up rather quickly to Mut in terms of single-target damage. (Perhaps Sub as well; Sub has been entirely ignored as a raiding spec nearly the entire expansion, but I haven't seen any evidence yet that its uselessness will continue in 5.4.)

Ergo, that first week or three of SoO might feel a little substandard as Combat, but if you're into that level of min-maxing you'd likely just be running as Mut anyway on a typical single-target fight, I'd imagine.

I'm not saying it's wonderful that so much of Combat's viability in competitive raiding appears to revolve around the relative power of its set bonuses and one particular trinket. But the spec could be in much worse shape.

Meanwhile, "dangerously close" is subjective, but I also think 12 days is still plenty of time for the class designers to work out critical balance issues if they're deemed important (and legitimate) enough.